Discussion
In this study, we aimed to explore whether baseline
vagally-mediated HRV (measured
via RMSSD) is a marker of the habitual tendency to ruminate, which is a
maladaptive thought process that makes individuals vulnerable to mental
disorders (measured by the RRS), in a large sample of healthy
individuals. Based on our data from over 1100 volunteers, no evidence is
found for the relationship between baseline
vagally-mediated HRV at rest and
rumination.
Reduced resting HRV has been reported in various forms of
psychopathology (Chalmers et al., 2014; Faurholt-Jepsen et al., 2017;
Tak et al., 2009), and the association between reduced HRV with trait
rumination has been reported in clinical and experimental studies
(Johnson et al., 2014; McLaughlin & Nolen-Hoeksema, 2011). For example,
a correlational study consisting of 117 women (55 recovered from
depression and 56 healthy controls) found a significant relation between
brooding, a maladaptive form of rumination, and HRV, such that women
reporting higher levels of brooding exhibited lower levels of HRV (Woody
et al., 2014). Another study monitored HRV in 52 healthy volunteers
throughout the day and found that the tendency to worry (another
perseverative cognition, highly correlated to rumination) was associated
with lower HRV (Brosschot et al., 2007; Fresco et al., 2002). In line
with these results, the seminal neurovisceral integration model proposes
that in healthy individuals, the prefrontal cortex is in control over
subcortical areas, resulting in tonic inhibitory control of the
parasympathetic branch of the ANS (i.e., higher HRV; Thayer & Lane,
2009). As a result, these neural and autonomic regulations result in
less viscous cycles of ruminative thoughts and turn higher resting HRV
(Song et al., 2022). However, in the current study, this association
between the tendency to ruminate and baseline vagally-mediated HRV
measured at rest cannot be observed in healthy individuals, even after
controlling for gender, age, and the level of depressive symptoms.
Moreover, Bayesian analyses showed a higher likelihood of no association
between HRV. Finally, our results exclude a non-linear relationship
between HRV with rumination and the level of depressive symptoms, which
is in line with a prior study that cannot find a linear or non-linear
effect of baseline resting HRV on positive emotion and depression
(Silvia et al., 2014).
A meta-analysis reviewing the relationship between physiological
concomitants with perseverative cognition found that maladaptive thought
processes (including rumination and worry) were associated with
decreased HRV in 8 correlational studies, but the literature was
insufficient to conclude whether this association existed specifically
for both traits (i.e., habitual tendency) and state (i.e., momentary)
rumination (Ottaviani et al., 2016). Two studies focused on the
relationship between the tendency to ruminate and HRV recovery in
children aged 7-14 and reported inconsistent results (Sample size: 100
and 103; Borelli et al., 2014; Gentzler et al., 2013). One study found a
negative correlation between resting HRV and emotion regulation, as
measured by the difficulties in emotion regulation scale, in 198
undergraduate students (Rankin et al., 2014). Only two studies described
in the previous paragraph specifically focused on trait rumination,
worry, and resting HRV and reported significant negative associations
(Brosschot et al., 2007; Woody et al., 2014). The remaining three
studies (of the 8 studies included in the meta-analysis) were all
related to instantaneous changes in HRV or state rumination and worry
showing significant results but in the context of laboratory-based
operation cannot generalize to the effects of chronic stress in everyday
life (Cash et al., 2013; Gazelle & Druhen, 2009; Mezulis et al., 2014).
Thus, existing studies either have focused on all-female samples and
include depression risk rather than completely healthy controls,
conclusions were drawn from young school children, or the sample size
was relatively low. In addition, although an overlap, there is still
distinguishing characteristic of rumination and worry (Nolen-Hoeksema et
al., 2008). Therefore, the finding of this study, based on a large
sample size and a young healthy population with a wider age range,
suggests that the tendency to ruminate is not associated with lower HRV
in healthy individuals.
Some characteristics of our study can, at least in part, explain the
null association between HRV and rumination. Although the participants
in the current dataset report a wide range of habitual tendencies
towards rumination, they are healthy young individuals without mental
diseases. Thus, it is possible that the association between resting HRV
and rumination is more robust in individuals who have impaired
parasympathetic control due to long-term stress exposure or who suffer
from stress-related mental disorders. As a systematic review suggests,
individuals who reported childhood adversity (retrospect of the past, a
ruminative thought) have no significant overall association with resting
HRV, whereas the clinical sample, as significant moderators, may have
small significant association under specific circumstances (Wesarg et
al., 2022). These might provide an explanation for the absent
association between trait rumination and resting HRV in the current
large sample of healthy individuals. Also, HRV was measured during a
resting period, and no stress or ruminative processes were induced.
Thus, another explanation could be that the negative correlation is more
related to state (i.e., at that moment) rumination and acute changes in
HRV under experimental induction paradigms rather than in a resting
phase (Ottaviani et al., 2016). In sum, based on our findings, resting
vagally-mediated HRV cannot be
regarded as an index of the tendency to ruminate, and thus a
vulnerability factor in trait rumination, in healthy individuals.
Overall, despite an absent correlation between baseline HRV measured
during rest, associations between other variables are in line with the
literature. The results of our large-scale cross-sectional dataset show
that the resting HRV of males was slightly higher than that of females
(Hamidovic et al., 2020). Furthermore, sex differences were found in the
rumination scores (females scoring higher as compared to males), but not
in the questionnaires regarding depressive symptoms. In addition, our
results also show that, based on a limited age range, there was a
significant negative correlation between age and rumination scores, as
well as age and BDI-II scores, indicating that with the increase in age,
people experience fewer symptoms of rumination and depression. This
observation is in line with prior studies reporting that older people
show a decrease in processing negative stimuli and thus show a positive
effect on emotion (Nashiro et al., 2012). Rumination and depression were
also highly correlated, as previous studies suggested that rumination is
a potential risk factor for depression and can prolong and intensify
depressive symptoms (Nolen-Hoeksema et al., 1997; Spasojević & Alloy,
2001). Overall, these findings indicate that the association between
rumination and related psychological constructs is consistent with those
reported in the previous literature, showing the validity of our data.
Accurate baseline measurements are critical to measuring the impact of
cognitive tasks or group designations. At the resting baseline phase,
participants usually sit with their knees at a 90° angle, both feet flat
on the floor, hands on thighs, and no psychological tasks. This protocol
involves inevitable mind wandering, disruptive thoughts, or difficulty
sitting still for some participants. A popular alternative to this
forced relaxation could be vanilla baseline, where participants must
perform a task that requires sustained attention but a minimal cognitive
load (such as reading a magazine or coloring). Importantly, our results
show that different baseline types influenced resting HRV, denoting that
the HRV values during a vanilla baseline were higher as compared to a
baseline in rest. Further studies should consider baseline type
depending on specific experimental tasks and purpose, while there is no
simple answer to defining an appropriate baseline measurement.
Some limitations of our study should be considered. It is important to
mention that the sex ratio in this study is not balanced and that sex
itself affects HRV, which may impede the broad applicability of our
results. Body mass index is also associated with HRV, but it was not
measured in some of our experiments. In addition, the measurement of
trait rumination and resting HRV was inconsistent in time and space. The
trait rumination questionnaire was generally completed online (before
the experiment), while HRV was measured in a laboratory environment, and
generally not on the same day. Moreover, as all the participants were
recruited in the same lab, we cannot rule out the possibility of
subjects that participated in multiple experiments. All the above
factors might contribute to the observed results.
To conclude, this is the first study using a large cross-sectional
dataset to examine the relationship between baseline resting HRV and
trait rumination in healthy individuals. The results indicate that there
is no association between HRV under a specific condition, namely
baseline resting, and the tendency to ruminate in daily life, based on a
sample of more than thousands of adults. Thus, vagally-mediated HRV at
rest could not be regarded as a marker of trait rumination in healthy
individuals.